Positive electrode active material, positive electrode, and lithium secondary battery containing the same

Abstract

To provide a bimodal-type positive electrode active material for remedying the low energy density per unit volume of an overlithiated lithium manganese-based oxide.SOLUTION: A first lithium manganese-based oxide and a second lithium manganese-based oxide are oxides in which a phase belonging to the C2 / m space group and a phase belonging to the R3-m space group are dissolved or complexed. The first lithium manganese-based oxide and the second lithium manganese-based oxide each independently comprise a secondary particle in at least one form selected from a secondary particle in the form of aggregated large-diameter primary particles and a secondary particle in the form of aggregated small-diameter primary particles. At least one selected from the first lithium manganese-based oxide and the second lithium manganese-based oxide comprises the secondary particle in the form of aggregated large-diameter primary particles.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a positive electrode active material, a positive electrode, and a lithium secondary battery containing the same, and more specifically, to a bimodal type positive electrode active material, a positive electrode, and a lithium secondary battery containing the same for improving the low energy density per unit volume of lithium-rich lithium manganese oxides. [Background technology]

[0002] Batteries store electricity by using electrochemically reactive materials at the positive and negative electrodes. A typical example of such a battery is the lithium-ion secondary battery, which stores electrical energy through the difference in chemical potential that occurs when lithium ions are intercalated / deintercalated at the positive and negative electrodes.

[0003] The lithium secondary battery is manufactured by using materials capable of reversible intercalation / deintercalation of lithium ions as positive electrode and negative electrode active materials, and by filling the space between the positive electrode and the negative electrode with an organic electrolyte or polymer electrolyte.

[0004] Typical materials used as positive electrode active materials in lithium secondary batteries include lithium composite oxides. These lithium composite oxides include LiCoO2, LiMn2O4, LiNiO2, LiMnO2, or oxides formed by the combination of Ni, Co, Mn, or Al.

[0005] Among the positive electrode active materials, LiCoO2 is the most widely used due to its excellent lifespan characteristics and charge / discharge efficiency. However, it has the disadvantage of being expensive due to the resource limitations of cobalt used as a raw material, thus limiting its price competitiveness.

[0006] Lithium manganese oxides such as LiMnO2 and LiMn2O4 have advantages such as excellent thermal safety and low cost, but they have the drawbacks of low capacity and poor high-temperature performance. On the other hand, LiNiO2-based cathode active materials exhibit high discharge capacity battery characteristics, but their synthesis is difficult due to cation mixing problems between Li and transition metals, which results in significant problems with their rate characteristics.

[0007] Furthermore, a large amount of Li by-products is generated depending on the degree of deepening of such cation mixing. These Li by-products mostly consist of LiOH and Li2CO3, which may cause gelation during the production of the positive electrode paste or generate gas due to repeated charging and discharging after electrode production. In addition, residual Li2CO3 among the Li by-products increases the swelling phenomenon of the cell, which reduces its lifespan characteristics.

[0008] Various candidate materials have been proposed to compensate for the shortcomings of these conventional cathode active materials.

[0009] As an example, research is being conducted to use lithium-rich lithium-manganese oxides, which contain an excess amount of manganese (Mn) among the transition metals, and whose lithium content exceeds the total content of the transition metals, as positive electrode active materials for lithium secondary batteries. Such lithium-rich lithium-manganese oxides are also called lithium-overlithiated layered oxides (OLOs).

[0010] While the aforementioned OLO has the advantage of theoretically exhibiting high capacity under high-voltage operating conditions, in reality, it has a disadvantage in that its electrical conductivity is relatively low due to the excess amount of Mn contained in the oxide, resulting in poor rate characteristics for lithium secondary batteries using OLO. When rate characteristics are low in this way, problems arise in which the charge / discharge capacity and life efficiency (cycle capacity retention rate) of the lithium secondary battery decrease during charge / discharge cycles.

[0011] In addition, due to the characteristics of the material, OLO has a disadvantage in that the porosity inside the particles is high, resulting in a low energy density per unit volume.

[0012] In order to solve the above problems, research on changing the composition of OLO has continued, but at present, such attempts have not reached the commercialization level.

Summary of the Invention

Problems to be Solved by the Invention

[0013] In the lithium secondary battery market, with the growth of lithium secondary batteries for electric vehicles playing a role as a market driver, the demand for the positive electrode active material used in lithium secondary batteries has also been continuously increasing.

[0014] For example, conventionally, from the perspective of ensuring safety, etc., lithium secondary batteries using lithium iron phosphate (LFP) have been mainly used, but recently, the use of nickel-based lithium composite oxides with a higher energy capacity per unit weight compared to LFP has been on the trend of expansion.

[0015] Also, recently, nickel-based lithium composite oxides mainly used as the positive electrode active material of high-capacity lithium secondary batteries essentially use ternary metal elements such as nickel, cobalt, and manganese or nickel, cobalt, and aluminum. However, cobalt not only has unstable supply and demand but is also excessively expensive compared to other raw materials, so there is a need for a new composition of positive electrode active material with a reduced cobalt content or without cobalt.

[0016] Considering these circumstances, lithium-rich lithium manganese oxides can meet the aforementioned market expectations, but they still have limitations in terms of electrochemical properties and stability, making them suitable as a substitute for commercially available ternary lithium composite oxides with nickel-cobalt-manganese (NCM) or nickel-cobalt-aluminum (NCA) compositions.

[0017] For example, as mentioned earlier, OLO has the disadvantage of having a low energy density per unit volume due to its material composition (containing excess lithium) and structural properties (high porosity within the particles).

[0018] However, the inventors have confirmed that by preparing the lithium manganese oxide by separating it into small and large particles, and then providing a bimodal type positive electrode active material as a mixture of the small and large particles, the low energy density per unit volume of lithium-rich lithium manganese oxide can be improved.

[0019] Thus, the present invention aims to provide a bimodal type cathode active material for improving the low energy density per unit volume of lithium-rich lithium manganese oxides.

[0020] Furthermore, the present invention aims to provide a positive electrode active material that can further improve the energy density per unit volume of a bimodal type positive electrode active material by including at least one of the small particles and large particles as a secondary particle from which the growth of the primary particle has been induced.

[0021] In particular, secondary particles in which the growth of primary particles contained in at least one of the small and large particles is induced can contribute to improving the energy density per unit volume of the bimodal type cathode active material by reducing the internal porosity of the secondary particles (porosity between primary particles).

[0022] Furthermore, secondary particles, which are formed by the growth of primary particles contained in at least one of the small and large particles, have a reduced specific surface area. This reduction in the specific surface area of ​​the secondary particles prevents a rapid decrease in battery performance due to side reactions during the initial battery reaction under high voltage conditions.

[0023] Furthermore, the present invention aims to provide a lithium secondary battery that, by using a positive electrode containing a bimodal type positive electrode active material as defined in this application, prevents a decrease in the electrochemical properties of the lithium secondary battery, including rate characteristics due to lithium and manganese present in excess in the OLO, and achieves high stability by reducing side reactions between the positive electrode active material and the electrolyte during high-voltage operation. [Means for solving the problem]

[0024] According to one aspect of the present invention for solving the above-mentioned technical problems, a bimodal type cathode active material is provided which includes a first lithium manganese-based oxide as small particles and a second lithium manganese-based oxide as large particles.

[0025] The first lithium manganese-based oxide and the second lithium manganese-based oxide constituting the bimodal type positive electrode active material are oxides in which a phase belonging to the C2 / m space group and a phase belonging to the R3-m space group are in solid solution or composite form.

[0026] Generally, commercially available ternary lithium composite oxides with nickel-cobalt-manganese (NCM) or nickel-cobalt-aluminum (NCA) compositions have a single phase belonging to the R3-m space group, whereas the lithium-rich lithium manganese oxides defined in this application are characterized by solid solution or composite formation of a phase belonging to the C2 / m space group and a phase belonging to the R3-m space group.

[0027] In one embodiment, the first lithium manganese oxide and the second lithium manganese oxide include secondary particles formed by the aggregation of a plurality of primary particles.

[0028] Specifically, the first lithium manganese oxide and the second lithium manganese oxide each independently include at least one form of secondary particles selected from a form in which multiple large-sized primary particles are aggregated and a form in which multiple small-sized primary particles are aggregated.

[0029] In this case, the energy density per unit volume of the bimodal type positive electrode active material can be improved by ensuring that at least one selected from the first lithium manganese oxide and the second lithium manganese oxide contains secondary particles in the form of aggregated large-particle primary particles.

[0030] In one embodiment, the first lithium manganese oxide may contain secondary particles in the form of aggregated large primary particles and secondary particles in the form of aggregated small primary particles in a weight ratio of 10:90 to 100:0.

[0031] Here, the average value of the short axis length of the large-sized primary particles is 100 nm or more and 500 nm or less, the average value of the short axis length of the small-sized primary particles is 50 nm or more and 300 nm or less, and the average value of the short axis length of the small-sized primary particles is smaller than the average value of the short axis length of the large-sized primary particles.

[0032] The average of the short-axis lengths of the primary particles can be measured from the SEM image of the secondary particles using the primary particles exposed on the surface of the secondary particles (for example, calculated from 20 primary particles selected in descending order of short-axis length from the primary particles exposed on the surface of the secondary particles using the SEM image of the secondary particles).

[0033] In one embodiment, the second lithium manganese oxide may contain secondary particles in the form of aggregated large primary particles and secondary particles in the form of aggregated small primary particles in a weight ratio of 10:90 to 100:0.

[0034] Here, the average value of the short axis length of the large-sized primary particles is 100 nm or more and 500 nm or less, the average value of the short axis length of the small-sized primary particles is 50 nm or more and 300 nm or less, and the average value of the short axis length of the small-sized primary particles is smaller than the average value of the short axis length of the large-sized primary particles.

[0035] The first lithium manganese oxide and the second lithium manganese oxide may each independently contain at least one selected from nickel, cobalt, and manganese.

[0036] More specifically, the first lithium manganese oxide and the second lithium manganese oxide can each be independently represented by the following chemical formula 1.

[0037] [Chemical formula 1] rLi2MnO 3-b′ X b′ (1-r)Li a M1 x M2 y M3 z O 2-b X b (Here, M1 is at least one selected from Ni and Mn. M2 is at least one selected from Ni, Mn, Co, Al, P, Nb, B, Si, Ti, Zr, Ba, K, Mo, Fe, Cu, Cr, Zn, Na, Ca, Mg, Pt, Au, Eu, Sm, W, Ce, V, Ta, Sn, Hf, Gd, and Nd. M3 is at least one selected from W, Mo, and Nb. M1 to M3 do not overlap with each other. X and X' are halogens capable of substituting at least a portion of the oxygen present in the lithium manganese oxide. 0 <r≦0.7、0<a≦1、0≦b≦0.1、0≦b′≦0.1、0<x≦1、0≦y<1、0≦z≦0.1および0<x+y+z≦1である。)

[0038] At least one selected from the first lithium manganese oxide and the second lithium manganese oxide may include secondary particles doped with at least one dopant selected from metal cation dopants and halogen anion dopants, or preferably, secondary particles in the form of aggregated multiple large-particle primary particles may be doped with at least one dopant selected from metal cation dopants and halogen anion dopants.

[0039] In this case, the metal cation dopant is at least one selected from W, Mo, and Nb, and the halogen anion dopant may be fluorine.

[0040] Furthermore, according to another aspect of the present invention, a positive electrode containing the above-described positive electrode active material is provided.

[0041] Furthermore, according to yet another aspect of the present invention, a lithium secondary battery is provided in which the above-described positive electrode is used. [Effects of the Invention]

[0042] According to the present invention, it is possible to improve upon the limitations of conventional lithium-rich lithium manganese oxides, which have various disadvantages in terms of electrochemical properties and / or stability when compared with commercially available ternary lithium composite oxides of nickel-cobalt-manganese (NCM) or nickel-cobalt-aluminum (NCA) composition.

[0043] Specifically, according to the present invention, by separating the lithium manganese oxide into small particles and large particles and then providing a bimodal type positive electrode active material as a mixture of the small and large particles, it is possible to partially improve the low energy density per unit volume of lithium-rich lithium manganese oxide.

[0044] Furthermore, according to the present invention, it is possible to further improve the energy density per unit volume of a bimodal type cathode active material by ensuring that at least one of the small particles and large particles includes secondary particles (secondary particles in which large-sized primary particles have aggregated) from which the growth of primary particles has been induced.

[0045] In particular, secondary particles in which large-sized primary particles, contained in at least one of the small and large particles, are aggregated can contribute to further improvement of the energy density per unit volume of bimodal type cathode active materials by reducing the internal porosity (porosity between primary particles).

[0046] Furthermore, secondary particles formed by the aggregation of large-diameter primary particles contained in at least one of the small and large particles have a reduced specific surface area, which prevents a rapid decrease in battery performance due to side reactions during the initial battery reaction under high voltage conditions.

[0047] For example, if the specific surface area of ​​small and / or large particles decreases, it is possible to reduce side reactions between the small and / or large particles and the electrolyte. In particular, OLOs such as lithium manganese oxides have the advantage of exhibiting high capacity under high-voltage operating conditions, but the possibility of side reactions occurring between the lithium manganese oxide and the electrolyte can be promoted as the operating voltage increases, so it is important to reduce side reactions between the lithium manganese oxide and the electrolyte.

[0048] Therefore, by reducing side reactions between small and / or large particles and the electrolyte, the stability and lifespan of lithium secondary batteries using this as a bimodal type positive electrode active material as defined in this application can be improved. In particular, a positive electrode active material in which side reactions with the electrolyte are suppressed can drive lithium secondary batteries at higher voltages.

[0049] Along with the effects described above, the specific effects of the present invention will be described below while explaining the specific matters for carrying out the invention. [Modes for carrying out the invention]

[0050] For the convenience of making the present invention easier to understand, certain terms are defined in this application. Unless otherwise specifically defined in this application, the scientific and technical terms used in this invention have meanings that are generally understood by those of ordinary skill in the art. Furthermore, unless otherwise specified in the context, singular terms should be understood to include their plural forms, and plural terms should be understood to include their singular forms.

[0051] The following describes in more detail some embodiments of the present invention, specifically positive electrode active materials containing lithium-rich lithium manganese oxides and lithium secondary batteries containing such positive electrode active materials.

[0052] positive electrode active material According to one aspect of the present invention, a positive electrode active material is provided which contains a lithium manganese oxide in which a phase belonging to the C2 / m space group and a phase belonging to the R3-m space group are in solid solution or composite.

[0053] Here, the lithium manganese oxide can be classified into a first lithium manganese oxide as small particles and a second lithium manganese oxide as large particles based on the average particle size, and the positive electrode active material defined in this application is a bimodal type positive electrode active material containing a first lithium manganese oxide as small particles and a second lithium manganese oxide as large particles.

[0054] The lithium manganese oxide contains at least lithium, nickel, and manganese. In this case, the lithium manganese oxide is also called an overlithiated layered oxide (OLO) when the lithium content present in the lithium manganese oxide is greater than the total content of other transition metals (generally, when the molar ratio of lithium to all other metal elements in the lithium manganese oxide (Li / Metal molar ratio) is greater than 1).

[0055] Furthermore, the lithium manganese oxide comprises at least one selected from nickel, cobalt, and manganese. That is, the first lithium manganese oxide and the second lithium manganese oxide each independently comprise at least one selected from nickel, cobalt, and manganese.

[0056] Generally, considering that commercially available ternary lithium composite oxides with nickel-cobalt-manganese (NCM) or nickel-cobalt-aluminum (NCA) compositions have a manganese content of 20 mol% or less in the total metal elements excluding lithium, the lithium manganese-based oxides have a relatively higher proportion of manganese (e.g., 50 mol% or more, preferably 55 mol% to 75 mol%) in the total metal elements compared to commercially available ternary lithium composite oxides.

[0057] Furthermore, considering that commercially available ternary lithium composite oxides with nickel-cobalt-manganese (NCM) or nickel-cobalt-aluminum (NCA) compositions have a nickel content of 60 mol% or more (80 mol% or more in the case of high-Ni types) among all metal elements excluding lithium, the lithium manganese-based oxide has a relatively lower proportion of nickel among all metal elements (for example, less than 50 mol%, preferably 25 mol% to 45 mol%) compared to commercially available ternary lithium composite oxides.

[0058] Another difference is that the Li / Metal molar ratio measured from lithium manganese oxides as defined in this application is greater than that of ternary lithium composite oxides such as nickel-cobalt-manganese (NCM) or nickel-cobalt-aluminum (NCA). For example, the Li / Metal molar ratio of ternary lithium composite oxides such as nickel-cobalt-manganese (NCM) or nickel-cobalt-aluminum (NCA) is close to 1. On the other hand, the Li / Metal molar ratio of lithium manganese oxides as defined in this application is greater than 1, preferably between 1.1 and 1.7.

[0059] Despite the aforementioned compositional differences, the lithium manganese-based oxide can also function as a composite metal oxide capable of lithium ion intercalation / deintercalation.

[0060] In one embodiment, the first lithium manganese oxide and the second lithium manganese oxide include secondary particles formed by the aggregation of a plurality of primary particles.

[0061] Specifically, the first lithium manganese oxide and the second lithium manganese oxide each independently include at least one form of secondary particles selected from a form in which multiple large-sized primary particles are aggregated and a form in which multiple small-sized primary particles are aggregated.

[0062] In this application, the small-particle primary particles and the large-particle primary particles can be classified based on the average value of the short axis length of the primary particles.

[0063] The terms "small diameter" and "large diameter," as used in this application, are relative concepts used to describe the size of primary particles that constitute secondary particles. Therefore, the terms "small diameter" and "large diameter" should be understood as having meanings distinct from "small particle" and "large particle," which are terms used to describe the size of secondary particles.

[0064] If the first lithium manganese oxide or the second lithium manganese oxide each contains secondary particles with different primary particle sizes, the primary particles that constitute secondary particles formed by the aggregation of primary particles having a relatively small average particle size can also be referred to as "small-particle primary particles." Conversely, the primary particles that constitute secondary particles formed by the aggregation of primary particles having a relatively large average particle size can also be referred to as "large-particle primary particles."

[0065] The small-particle primary particles and the large-particle primary particles may have a variety of shapes within the range defined in this application.

[0066] Conventional lithium-rich lithium manganese oxides typically have a secondary particle form in which multiple primary particles aggregate, generally with an average particle size of several to tens of nanometers.

[0067] On the other hand, the primary particles constituting the lithium manganese-based oxide as defined in this application may have an average particle size of 0.05 μm to 5 μm, preferably 0.05 μm to 1.0 μm. More specifically, the small-particle primary particles may have an average particle size of 0.05 μm to 5 μm, preferably 0.05 μm to 1.0 μm, more preferably 0.10 μm to 0.50 μm, and the large-particle primary particles may have an average particle size of 0.05 μm to 5 μm, preferably 0.05 μm to 1.0 μm, more preferably 0.15 μm to 0.75 μm.

[0068] For example, if the first lithium manganese oxide simultaneously contains secondary particles in which multiple large-sized primary particles are aggregated and secondary particles in which multiple small-sized primary particles are aggregated, the average particle size of the large-sized primary particles constituting the secondary particles in which multiple large-sized primary particles are aggregated is greater than the average particle size of the small-sized primary particles constituting the secondary particles in which multiple small-sized primary particles are aggregated.

[0069] The major axis length of the primary particle, the minor axis length of the primary particle, the ratio of the major axis length to the minor axis length of the primary particle (major axis length / minor axis length), and the average particle size of the primary particle ([major axis length + minor axis length] / 2) can be calculated as the average value of the major axis length and minor axis length of the primary particle exposed on the surface of the secondary particle, respectively.

[0070] For example, the average value of the results measured from all primary particles exposed on the surface of the secondary particle, or the average value of the results measured from a selection of primary particles exposed on the surface of the secondary particle (for example, calculated from a selection of primary particles (e.g., 10 primary particles, 20 primary particles, etc.) selected from the SEM image of the secondary particle in order of decreasing short-axis length) can be used.

[0071] When the average particle size of the primary particles is less than 0.1 μm, the specific surface area of ​​the lithium manganese oxide (secondary particles) composed of the primary particles is relatively large. In this case, there is a higher possibility that the lithium manganese oxide and the electrolyte will undergo a side reaction during storage or operation of the lithium secondary battery.

[0072] On the other hand, if the average particle size of the primary particles is greater than 5 μm, the growth of the primary particles is excessively induced, which lengthens the diffusion pathway of lithium ions within the primary particles. When the diffusion pathway of lithium ions within the primary particles is excessively long, the mobility of lithium ions within the primary particles and the diffusivity of lithium ions mediated by the primary particles decrease, which increases the resistance of the lithium manganese oxide (secondary particles) composed of the primary particles.

[0073] The first lithium manganese oxide may consist of only at least one form of secondary particles selected from secondary particles in which a plurality of small-sized primary particles are aggregated, or it may consist only of secondary particles in which a plurality of large-sized primary particles are aggregated, or it may consist of secondary particles in which a plurality of large-sized primary particles are aggregated and secondary particles in which a plurality of small-sized primary particles are aggregated.

[0074] The second lithium manganese oxide may consist of only at least one form of secondary particles selected from secondary particles in which a plurality of small-sized primary particles are aggregated, or it may consist only of secondary particles in which a plurality of large-sized primary particles are aggregated, or it may consist of secondary particles in which a plurality of large-sized primary particles are aggregated and secondary particles in which a plurality of small-sized primary particles are aggregated.

[0075] In this case, at least one selected from the first lithium manganese oxide and the second lithium manganese oxide contains secondary particles in the form of aggregated large-particle primary particles, thereby improving the energy density per unit volume of the bimodal type positive electrode active material as defined in this application.

[0076] In one embodiment, the first lithium manganese oxide may contain secondary particles in the form of aggregated large primary particles and secondary particles in the form of aggregated small primary particles in a weight ratio of 10:90 to 100:0. In this case, the average value of the short axis length of the small primary particles is smaller than the average value of the short axis length of the large primary particles.

[0077] The average value of the short axis length of the large-sized primary particles may be 100 nm or more and 500 nm or less, preferably 150 nm or more and 400 nm or less. As mentioned above, the average value of the short axis length of the large-sized primary particles can be the average value of the short axis lengths measured from all primary particles exposed on the surface of secondary particles in a form in which multiple large-sized primary particles are aggregated, or from a selection of primary particles (for example, 10 primary particles, 20 primary particles, etc.) that are selected in order of short axis length from among the primary particles exposed on the surface of secondary particles in a form in which multiple large-sized primary particles are aggregated, in descending order of short axis length.

[0078] When the average short-axis length of the large-sized primary particles is less than 100 nm, the particle size is substantially similar to that of the small-sized primary particles. Therefore, even when using secondary particles formed by the aggregation of multiple large-sized primary particles and secondary particles formed by the aggregation of multiple small-sized primary particles, the improvement in energy density per unit volume is only slight.

[0079] On the other hand, the fact that the average short-axis length of the large-sized primary particles is greater than 500 nm means that the growth of the large-sized primary particles was generally excessively induced. Unnecessary promotion of the growth of the primary particles may reduce the mobility of lithium ions within the primary particles and the diffusivity of lithium ions mediated by the primary particles.

[0080] Furthermore, from the viewpoint of reducing the deviation in the short axis length of the large-diameter primary particles exposed on the surface of the secondary particles, the minimum value of the short axis length measured for the large-diameter primary particles exposed on the surface of the secondary particles is 50 nm or more, preferably 80 nm or more, and the maximum value of the short axis length measured for the large-diameter primary particles exposed on the surface of the secondary particles may be 1 μm or less, preferably 750 nm or less, and more preferably 500 nm or less. The smaller the deviation in the short axis length of the large-diameter primary particles exposed on the surface of the secondary particles, the more uniformly the surface kinetics of the secondary particles can be maintained.

[0081] The average value of the short axis length of the small-sized primary particles may be 50 nm or more and 300 nm or less, preferably 60 nm or more and 200 nm or less. As mentioned above, the average value of the short axis length of the small-sized primary particles can be the average value of the short axis lengths measured from all primary particles exposed on the surface of secondary particles in a form in which multiple small-sized primary particles are aggregated, or from multiple primary particles (for example, 10 primary particles, 20 primary particles, etc.) selected in order of short axis length from among the primary particles exposed on the surface of secondary particles in a form in which multiple small-sized primary particles are aggregated, in descending order of short axis length.

[0082] When the average value of the short axis length of the small-particle primary particles is less than 50 nm, the likelihood of a side reaction occurring between the lithium manganese oxide and the electrolyte during storage or operation of the lithium secondary battery increases as the specific surface area of ​​the secondary particles composed of the small-particle primary particles becomes relatively larger.

[0083] On the other hand, if the average value of the short axis length of the small-sized primary particles is greater than 300 nm, the particle size is substantially similar to that of the large-sized primary particles. Therefore, even if secondary particles formed by the aggregation of multiple large-sized primary particles and secondary particles formed by the aggregation of multiple small-sized primary particles are used, the improvement in energy density per unit volume is only slight.

[0084] Furthermore, from the viewpoint of reducing the deviation in the short axis length of the small-diameter primary particles exposed on the surface of the secondary particles, the minimum value of the short axis length measured for the small-diameter primary particles exposed on the surface of the secondary particles may be 20 nm or more, preferably 40 nm or more, and the maximum value of the short axis length measured for the small-diameter primary particles exposed on the surface of the secondary particles may be 500 nm or less, preferably 300 nm or less. The smaller the deviation in the short axis length of the small-diameter primary particles exposed on the surface of the secondary particles, the more uniformly the surface kinetics of the secondary particles can be maintained.

[0085] Despite the difference in primary particle size, the difference in average particle size between secondary particles formed by the aggregation of multiple large-sized primary particles and secondary particles formed by the aggregation of multiple small-sized primary particles does not need to be significant. For example, if the number of primary particles constituting secondary particles formed by the aggregation of multiple large-sized primary particles is less than the number of primary particles constituting secondary particles formed by the aggregation of multiple small-sized primary particles, the average particle sizes of secondary particles formed by the aggregation of multiple large-sized primary particles and secondary particles formed by the aggregation of multiple small-sized primary particles may be similar.

[0086] In other words, the secondary particles formed by the aggregation of multiple large-sized primary particles and the secondary particles formed by the aggregation of multiple small-sized primary particles that constitute the first lithium manganese-based oxide have an average particle size of 2 μm to 5 μm.

[0087] Furthermore, it is preferable that the interparticle porosity between primary particles, measured from a cross-sectional SEM image of secondary particles formed by the aggregation of large-particle primary particles, is smaller than the interparticle porosity between primary particles, measured from a cross-sectional SEM image of secondary particles formed by the aggregation of small-particle primary particles.

[0088] Specifically, the interparticle porosity between the primary particles, measured from a cross-sectional SEM image of the secondary particles in which the large-particle-sized primary particles have aggregated, may be 10% or less, preferably 5% or less.

[0089] Furthermore, if we define r as the distance from the center of the secondary particle to the surface of the secondary particle, as determined from the cross-sectional SEM image of the secondary particle, and define the region between 0.5r and 1.0r from the center of the secondary particle as the outer bulk region, then the porosity within the outer bulk region measured from the cross-sectional SEM image of the secondary particle in the form of aggregated large-particle primary particles may be smaller than the porosity within the outer bulk region measured from the cross-sectional SEM image of the secondary particle in the form of aggregated small-particle primary particles.

[0090] Specifically, the porosity within the outer bulk region, measured from a cross-sectional SEM image of secondary particles in the form of aggregated large-particle primary particles, may be 1% or less, or 0.5% or less.

[0091] As the voids between the large-sized primary particles within the aggregated secondary particles decrease, the energy density per unit volume of the first lithium manganese oxide, and consequently the bimodal type positive electrode active material containing the first lithium manganese oxide, can be improved.

[0092] In other words, in a bimodal type positive electrode active material, by changing some of the secondary particles in the form of aggregated small primary particles constituting the first lithium manganese-based oxide to secondary particles in the form of aggregated large primary particles, or by making the first lithium manganese-based oxide consist only of secondary particles in the form of aggregated large primary particles, the energy density per unit volume can be further improved compared to a bimodal positive electrode active material in which small and large particles simply coexist.

[0093] In one embodiment, the second lithium manganese oxide may contain secondary particles in the form of aggregated large primary particles and secondary particles in the form of aggregated small primary particles in a weight ratio of 10:90 to 100:0. In this case, the average value of the short axis length of the small primary particles is smaller than the average value of the short axis length of the large primary particles.

[0094] The average value of the short axis length of the large-sized primary particles may be 100 nm or more and 500 nm or less, preferably 130 nm or more and 400 nm or less. As mentioned above, the average value of the short axis length of the large-sized primary particles can be the average value of the short axis lengths measured from all primary particles exposed on the surface of secondary particles in a form in which multiple large-sized primary particles are aggregated, or from a selection of primary particles (for example, 10 primary particles, 20 primary particles, etc.) that are exposed on the surface of secondary particles in a form in which multiple large-sized primary particles are aggregated, selected in order of decreasing short axis length.

[0095] When the average short-axis length of the large-sized primary particles is less than 100 nm, the particle size is substantially similar to that of the small-sized primary particles. Therefore, even when using secondary particles formed by the aggregation of multiple large-sized primary particles and secondary particles formed by the aggregation of multiple small-sized primary particles, the improvement in energy density per unit volume is only slight.

[0096] On the other hand, the fact that the average short-axis length of the large-sized primary particles is greater than 500 nm means that the growth of the large-sized primary particles was generally excessively induced. As the growth of the primary particles is unnecessarily promoted, there is a risk that the mobility of lithium ions within the primary particles and the diffusivity of lithium ions mediated by the primary particles will decrease.

[0097] Furthermore, from the viewpoint of reducing the deviation in the short axis length of the large-diameter primary particles exposed on the surface of the secondary particles, the minimum value of the short axis length measured for the large-diameter primary particles exposed on the surface of the secondary particles is 50 nm or more, preferably 75 nm or more, and the maximum value of the short axis length measured for the large-diameter primary particles exposed on the surface of the secondary particles may be 1 μm or less, preferably 750 nm or less, more preferably 500 nm or less. The smaller the deviation in the short axis length of the large-diameter primary particles exposed on the surface of the secondary particles, the more uniformly the surface kinetics of the secondary particles can be maintained.

[0098] The average value of the short axis length of the small-sized primary particles may be 50 nm or more and 300 nm or less, preferably 60 nm or more and 200 nm or less. As mentioned above, the average value of the short axis length of the small-sized primary particles can be the average value of the short axis lengths measured from all primary particles exposed on the surface of secondary particles in a form in which multiple small-sized primary particles are aggregated, or from a selection of primary particles (for example, 10 primary particles, 20 primary particles, etc.) that are selected in order of short axis length from among the primary particles exposed on the surface of secondary particles in a form in which multiple small-sized primary particles are aggregated, in descending order of short axis length.

[0099] When the average value of the short axis length of the small-particle primary particles is less than 50 nm, the likelihood of a side reaction occurring between the lithium manganese oxide and the electrolyte during storage or operation of the lithium secondary battery increases as the specific surface area of ​​the secondary particles composed of the small-particle primary particles becomes relatively larger.

[0100] On the other hand, if the average value of the short axis length of the small-sized primary particles is greater than 300 nm, the particle size is substantially similar to that of the large-sized primary particles. Therefore, even if secondary particles formed by the aggregation of multiple large-sized primary particles and secondary particles formed by the aggregation of multiple small-sized primary particles are used, the improvement in energy density per unit volume is only slight.

[0101] Furthermore, from the viewpoint of reducing the deviation in the short axis length of the small-diameter primary particles exposed on the surface of the secondary particles, the minimum value of the short axis length measured for the small-diameter primary particles exposed on the surface of the secondary particles may be 20 nm or more, preferably 40 nm or more, and the maximum value of the short axis length measured for the small-diameter primary particles exposed on the surface of the secondary particles may be 500 nm or less, preferably 300 nm or less. The smaller the deviation in the short axis length of the small-diameter primary particles exposed on the surface of the secondary particles, the more uniformly the surface kinetics of the secondary particles can be maintained.

[0102] Despite the difference in primary particle size, the difference in average particle size between secondary particles formed by the aggregation of multiple large-sized primary particles and secondary particles formed by the aggregation of multiple small-sized primary particles does not need to be significant. For example, if the number of primary particles constituting secondary particles formed by the aggregation of multiple large-sized primary particles is less than the number of primary particles constituting secondary particles formed by the aggregation of multiple small-sized primary particles, the average particle sizes of secondary particles formed by the aggregation of multiple large-sized primary particles and secondary particles formed by the aggregation of multiple small-sized primary particles may be similar.

[0103] Furthermore, it is preferable that the interparticle porosity between primary particles, measured from a cross-sectional SEM image of secondary particles formed by the aggregation of large-particle primary particles, is smaller than the interparticle porosity between primary particles, measured from a cross-sectional SEM image of secondary particles formed by the aggregation of small-particle primary particles.

[0104] Specifically, the interparticle porosity between the primary particles, measured from a cross-sectional SEM image of the secondary particles in which the large-particle-sized primary particles have aggregated, may be 15% or less, preferably 10% or less, and more preferably 5% or less.

[0105] Furthermore, if we define r as the distance from the center of the secondary particle to the surface of the secondary particle, as determined from the cross-sectional SEM image of the secondary particle, and define the region between 0.5r and 1.0r from the center of the secondary particle as the outer bulk region, then the porosity within the outer bulk region measured from the cross-sectional SEM image of the secondary particle in the form of aggregated large-particle primary particles may be smaller than the porosity within the outer bulk region measured from the cross-sectional SEM image of the secondary particle in the form of aggregated small-particle primary particles.

[0106] Specifically, the porosity within the outer bulk region, measured from a cross-sectional SEM image of secondary particles in the form of aggregated large-particle primary particles, may be 1% or less, or 0.5% or less.

[0107] As the voids between the large-sized primary particles within the aggregated secondary particles decrease, the energy density per unit volume of the second lithium manganese oxide, and consequently the bimodal type cathode active material containing the second lithium manganese oxide, can be improved.

[0108] In other words, by changing some of the secondary particles in the bimodal type positive electrode active material, which are composed of aggregated small primary particles, to secondary particles composed of aggregated large primary particles, or by making the second lithium manganese-based oxide consist only of secondary particles composed of aggregated large primary particles, the energy density per unit volume can be further improved compared to a bimodal positive electrode active material in which small and large particles coexist.

[0109] By making the difference between the average particle size of the second lithium manganese-based oxide and the average particle size of the first lithium manganese-based oxide 3 μm or more, preferably 4 μm or more, the energy density per unit volume of the bimodal cathode active material can be increased.

[0110] In this case, the average particle size calculated as the average value of the length in the long axis direction and the length in the short axis direction of the first lithium manganese oxide ([long axis length + short axis length] / 2) may be 2 μm to 5 μm, and the average particle size calculated as the average value of the length in the long axis direction and the length in the short axis direction of the second lithium manganese oxide ([long axis length + short axis length] / 2) may be 6 μm to 14 μm.

[0111] As stated above, the terms "small particle" and "large particle" used in this application are relative concepts used to describe secondary particle sizes. In this application, small particles should be understood to refer to first lithium manganese oxides, and large particles to second lithium manganese oxides.

[0112] Furthermore, in order to optimize the energy density per unit volume of the bimodal cathode active material, it is preferable that the first lithium manganese-based oxide and the second lithium manganese-based oxide are present in the cathode active material in a weight ratio of 10:90 to 80:20.

[0113] The first lithium manganese-based oxide may exist in a form filled in the voids between the second lithium manganese-based oxides, may adhere to the surface of the second lithium manganese-based oxides, or may also exist in a form in which the first lithium manganese-based oxides aggregate together.

[0114] When the proportion of the first lithium manganese-based oxide is excessively small compared to the second lithium manganese-based oxide in the positive electrode active material, it is difficult for the first lithium manganese-based oxide to be sufficiently filled in the voids formed by the second lithium manganese-based oxide.

[0115] On the other hand, when the proportion of the first lithium manganese-based oxide is excessively large compared to the second lithium manganese-based oxide in the positive electrode active material, the energy density per unit volume of the positive electrode active material may decrease.

[0116] The first lithium manganese-based oxide and the second lithium manganese-based oxide defined in the present application may each independently be represented by the following Chemical Formula 1.

[0117] [Chemical Formula 1] rLi2MnO 3-b′ X b′ ·(1-r)Li a M1 x M2 y M3 z O 2-b X b Here, M1 is at least one selected from Ni and Mn, M2 is at least one selected from Ni, Mn, Co, Al, P, Nb, B, Si, Ti, Zr, Ba, K, Mo, Fe, Cu, Cr, Zn, Na, Ca, Mg, Pt, Au, Eu, Sm, W, Ce, V, Ta, Sn, Hf, Gd, and Nd, M3 is at least one selected from W, Mo, and Nb, M1 to M3 do not overlap with each other, X and X' are halogens capable of substituting at least a portion of the oxygen present in the lithium manganese oxide. 0 <r≦0.7、0<a≦1、0≦b≦0.1、0≦b′≦0.1、0<x≦1、0≦y<1、0≦z≦0.1および0<x+y+z≦1である。

[0118] In the aforementioned chemical formula 1, if M1 is Ni, M2 may include Mn, and if M1 is Mn, M2 may include Ni. Also, if M1 is Ni and Mn, M2 may be absent, or if present, it may be other elements other than Ni and Mn.

[0119] The types of halogens that can be used for X and X' are determined by referring to the periodic table, and can be F, Cl, Br and / or I, and preferably F.

[0120] In one embodiment, in order to synthesize secondary particles in the form of aggregated large-particle primary particles, a method can be used in which the oxide precursor of the lithium manganese-based oxide is doped with a metal cation dopant and / or halogen anion dopant while inducing the growth of the primary particles during the calcination of the oxide precursor.

[0121] In this case, the metal cation dopant may contain at least one selected from W, Mo, and Nb, and the halogen anion dopant may contain fluorine.

[0122] In the aforementioned chemical formula 1, the metal cation dopant is represented by M3, and the halogen anion dopant is represented by X.

[0123] As described above, when doping with halogens to induce the growth of the primary particles constituting the lithium manganese oxide, preferably, at least a portion of the oxygen present in the lithium manganese oxide may be replaced with halogens.

[0124] When using an over-calcination method in which the primary particles constituting the lithium manganese oxide are heat-treated at a relatively high temperature without halogen doping to induce crystal growth or particle growth, the primary particles can grow, but there is a possibility that damage may occur to the crystal structure of the primary particles, leading to premature degradation of the positive electrode active material.

[0125] When fluorine is used as an anion dopant during the growth of the primary particles, the growth of the primary particles can be induced within a range that mitigates the decrease in the diffusibility of lithium ions mediated by the primary particles.

[0126] For fluorine doping of the primary particles, at least one anionic dopant selected from LiF, MgF2, HF, F2, XeF2, TbF4, CeF4, CoF3, AgF2, MoF3, AgF, CuF2, FeF3, CuF, VF3, CrF3, ZrF4, BaF2, CaF2, AlF3, NH4F, CeF3, and CsF, preferably at least one anionic dopant selected from LiF and MgF2, can be used.

[0127] When an M3-containing dopant is further used in addition to the aforementioned anion dopant to induce simultaneous doping during the calcination of the oxide precursor, the primary particles can grow into a near-spherical shape instead of growing into a plate-like shape.

[0128] In the M3-containing dopant, at least one selected from hydroxides, oxides, carbonides, nitrides, sulfides, and phosphorides containing at least one element selected from tungsten, molybdenum, and niobium can be used.

[0129] As described above, when the anion dopant and the M3-containing dopant are used in combination during the calcination of the oxide precursor to simultaneously dope the primary particles and induce the growth of the primary particles, the shape of the primary particles approaches a spherical shape, and at the same time, the porosity of the secondary particles formed by the aggregation of the primary particles can be reduced.

[0130] By reducing the porosity within the lithium manganese oxide, the capacity per unit volume can be increased, thereby compensating for the insufficient electrochemical properties of the positive electrode active material containing the lithium manganese oxide.

[0131] Furthermore, when the aforementioned anion dopant and M3-containing dopant are used in combination during the calcination of the oxide precursor to simultaneously dope the primary particles and induce the growth of the primary particles, it is particularly effective in reducing the porosity on the surface of the secondary particles.

[0132] The lithium manganese oxide represented by chemical formula 1 may selectively contain cobalt. When the lithium manganese oxide contains cobalt, the mole fraction of cobalt relative to the total number of moles of metal elements in the lithium manganese oxide may be 20% or less, preferably 15% or less, and more preferably 10% or less. In other cases, the lithium manganese oxide represented by chemical formula 1 may have a cobalt-free composition.

[0133] The Li / Metal molar ratio measured from the lithium manganese oxide represented by the chemical formula 1 is greater than 1, and preferably between 1.1 and 1.7. When the Li / Metal molar ratio measured from the lithium manganese oxide is greater than 1, it is possible to form a lithium-rich lithium manganese oxide. Furthermore, in order for the lithium manganese oxide to appropriately form a solid solution in which a phase belonging to the C2 / m space group and a phase belonging to the R3-m space group are solidly dissolved or composited, and at the same time to exhibit high capacity under high-voltage operating conditions, the Li / Metal molar ratio of the lithium manganese oxide is preferably between 1.1 and 1.7.

[0134] Furthermore, in order to appropriately form a solid solution in which the phase belonging to the C2 / m space group and the phase belonging to the R3-m space group are in solid solution or composite form, it is preferable that the manganese content in the total metal elements excluding lithium present in the lithium manganese oxide represented by chemical formula 1 is 50 mol% or more. In order for the lithium manganese oxide to have OLO characteristics that exhibit high capacity under high voltage operating conditions, it is more preferable that the manganese content in the total metal elements excluding lithium present in the lithium manganese oxide is 50 mol% or more and less than 80 mol%, and even more preferable that it is 55 mol% to 75 mol%. If the manganese content in the lithium manganese oxide exceeds 80 mol%, a phase transition may occur due to the movement of transition metals (especially manganese) within the lithium manganese oxide during chemical formation and / or operation of the lithium secondary battery. Such a phase transition forms a spinel phase, and the spinel phase acting as an impurity in the lithium manganese oxide can induce a decrease in charge / discharge capacity or voltage decay during the charge / discharge cycle of the lithium secondary battery.

[0135] In order to properly form a solid solution in which a phase belonging to the C2 / m space group and a phase belonging to the R3-m space group are in solid solution or composite, it is preferable that the nickel content in the total metal elements excluding lithium present in the lithium manganese oxide represented by chemical formula 1 is less than 50 mol%.

[0136] When the nickel content in the lithium manganese oxide is 50 mol% or more, the C2 / m phase may not form sufficiently, or the phase belonging to the C2 / m space group and the phase belonging to the R3-m space group may not form a sufficient solid solution, which can cause phase separation during conversion and / or operation of the lithium secondary battery.

[0137] Generally, commercially available ternary lithium composite oxides with nickel-cobalt-manganese (NCM) or nickel-cobalt-aluminum (NCA) compositions have a single-phase phase belonging to the R3-m space group.

[0138] On the other hand, lithium-rich lithium manganese oxides represented by chemical formula 1 consist of an oxide of a phase belonging to the C2 / m space group represented by rLi2MnO3 (hereinafter referred to as the "C2 / m phase") and (1-r)Li a M1 x M2 y M3 z O 2-b X b The oxides of the phases belonging to the R3-m space group represented by (hereinafter referred to as "R3-m phase") exist as a solid solution or composite oxide. For example, the lithium manganese-based oxide may exist in a state in which the oxide of the C2 / m phase and the oxide of the R3-m phase form a solid solution.

[0139] In this case, composite oxides in which a phase belonging to the C2 / m space group and a phase belonging to the R3-m space group are simply physically and / or chemically bonded or attached do not fall under the definition of a solid solution as defined in this application.

[0140] For example, a composite oxide having a phase belonging to the C2 / m space group, obtained by mixing a metal oxide having a phase belonging to the C2 / m space group with a metal oxide having a phase belonging to the R3-m space group, and having the surface coated with a metal oxide having a phase belonging to the R3-m space group, does not fall under the definition of a solid solution as defined in this application.

[0141] In the lithium manganese oxide represented by the chemical formula 1, if r exceeds 0.7, the proportion of Li2MnO3, which is the C2 / m phase oxide, in the lithium manganese oxide becomes excessively high. This can lead to an excessively high manganese content in the positive electrode active material, potentially reducing the discharge capacity. In other words, in order to sufficiently activate the C2 / m phase oxide, which has relatively high resistance, in the lithium manganese oxide and improve the surface kinetics, it is preferable that the R3-m phase oxide be present in a predetermined proportion or higher.

[0142] Lithium-ion rechargeable battery According to another aspect of the present invention, a positive electrode can be provided that includes a positive electrode current collector and a layer of the positive electrode active material formed on the positive electrode current collector. Here, the positive electrode active material layer may contain a lithium manganese-based oxide according to the various embodiments of the present invention described above as the positive electrode active material.

[0143] Therefore, a detailed explanation of lithium manganese oxides will be omitted, and only the remaining components not mentioned above will be described below. Also, for convenience, the lithium manganese oxides mentioned above will be referred to as the positive electrode active material below.

[0144] The positive electrode current collector is not particularly limited as long as it does not induce chemical changes in the battery and is conductive. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc. may be used. The positive electrode current collector may also have a thickness of 3 to 500 μm, and fine irregularities can be formed on the surface of the current collector to increase the adhesion strength of the positive electrode active material. For example, it may be used in various forms such as film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0145] The positive electrode active material layer may be manufactured by applying a positive electrode slurry composition, which includes a conductive material and, if necessary, a binder, together with the positive electrode active material, to the positive electrode current collector.

[0146] In this case, the positive electrode active material may be present in an amount of 80 wt% to 99 wt%, more specifically, 85 wt% to 98.5 wt%, relative to the total weight of the positive electrode active material layer. When present within this content range, excellent capacity characteristics can be observed, but the material is not necessarily limited to this range.

[0147] The conductive material is used to impart conductivity to the electrodes and can be used without particular limitations as long as it has electronic conductivity without causing chemical changes in the battery it is configured in. Specific examples include graphite such as natural graphite or artificial graphite, carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber, metal powders or metal fibers such as copper, nickel, aluminum, and silver, conductive whiskers such as zinc oxide and potassium titanate, conductive metal oxides such as titanium oxide, or conductive polymers such as polyphenylene derivatives. One of these may be used alone or a mixture of two or more. The conductive material may be included in an amount of 0.1 wt% to 15 wt% relative to the total weight of the positive electrode active material layer.

[0148] The binder plays a role in improving the adhesion between positive electrode active material particles and the adhesion between the positive electrode active material and the current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, of which one or more may be used. The binder may be included in an amount of 0.1 to 15 wt% relative to the total weight of the positive electrode active material layer.

[0149] The positive electrode may be manufactured by a conventional positive electrode manufacturing method, except that the positive electrode active material is used. Specifically, the positive electrode may be manufactured by applying a positive electrode slurry composition, prepared by dissolving or dispersing the positive electrode active material and selectively a binder and conductive material in a solvent, onto a positive electrode current collector, followed by drying and rolling.

[0150] The solvent may be any solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and one or more of these may be used individually or in mixtures of two or more. The amount of solvent used should be such that it dissolves or disperses the cathode active material, conductive material, and binder, and then provides a viscosity that allows for excellent thickness uniformity during coating for cathode manufacturing, taking into consideration the coating thickness and production yield of the slurry.

[0151] In other embodiments, the positive electrode may be manufactured by casting the positive electrode slurry composition onto a separate support, peeling it off the support, and then laminating the resulting film onto the positive electrode current collector.

[0152] Furthermore, according to yet another aspect of the present invention, an electrochemical element including the aforementioned positive electrode may be provided. Specifically, the electrochemical element may be a battery, a capacitor, or the like, and more specifically, a lithium secondary battery.

[0153] The lithium secondary battery may specifically include a positive electrode, a negative electrode positioned opposite the positive electrode, and a separator and electrolyte interposed between the positive and negative electrodes. Here, since the positive electrode is as described above, for convenience, a detailed explanation will be omitted, and only the remaining components not mentioned above will be described in detail below.

[0154] The lithium secondary battery may further selectively include a battery container for housing the electrode assembly comprising the positive electrode, the negative electrode, and the separator, and a sealing member for sealing the battery container.

[0155] The negative electrode may include a negative electrode current collector and a negative electrode active material layer located on the negative electrode current collector.

[0156] The negative electrode current collector is not particularly limited as long as it has high conductivity without inducing chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloy may be used. The negative electrode current collector may also typically have a thickness of 3 μm to 500 μm, and, similar to the positive electrode current collector, fine irregularities can be formed on the surface of the current collector to strengthen the bonding force of the negative electrode active material. For example, it may be used in various forms such as film, sheet, foil, net, porous material, foam, and nonwoven fabric.

[0157] The negative electrode active material layer may be manufactured by applying a negative electrode slurry composition, which includes the negative electrode active material together with a conductive material and, if necessary, a selective binder, to the negative electrode current collector.

[0158] As the negative electrode active material, a compound capable of reversible intercalation and deintercalation of lithium may be used. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic compounds capable of alloying with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys; and SiO2. βExamples include lithium-doped and dedoped metal oxides such as (0<β<2), SnO2, vanadium oxide, and lithium vanadium oxide, or composites containing the metallic compound and carbonaceous material, such as Si-C composites or Sn-C composites. One or more of these mixtures may be used. A metallic lithium thin film may also be used as the negative electrode active material. Furthermore, low-crystalline carbon and high-crystalline carbon may all be used as the carbon material. Typical examples of low-crystalline carbon include soft carbon and hard carbon, while typical examples of high-crystalline carbon include amorphous, plate-like, flaky, spherical, or fibrous natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, meso-carbon microbeads, mesophase pitches, and high-temperature calcined carbon such as petroleum or coal tar pitch-derived cokes.

[0159] The aforementioned negative electrode active material may be present in an amount of 80 wt% to 99 wt% based on the total weight of the negative electrode active material layer.

[0160] The binder may be added in an amount of 0.1 wt% to 10 wt% based on the total weight of the negative electrode active material layer, as a component that assists in bonding between the conductive material, active material, and current collector. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, nitrile-butadiene rubber, fluororubber, and various copolymers thereof.

[0161] The conductive material may be added as a component to further improve the conductivity of the negative electrode active material, in an amount of 10 wt% or less, preferably 5 wt% or less, based on the total weight of the negative electrode active material layer. Such a conductive material is not particularly limited as long as it has conductivity without inducing a chemical change in the battery, and for example, graphite such as natural graphite or artificial graphite, carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black, conductive fibers such as carbon fibers and metal fibers, metal powders such as carbon fluoride, aluminum, and nickel powder, conductive whiskers such as zinc oxide and potassium titanate, conductive metal oxides such as titanium oxide, and conductive materials such as polyphenylene derivatives may be used.

[0162] In one embodiment, the negative electrode active material layer may be manufactured by coating a negative electrode slurry composition, which is prepared by dissolving or dispersing a negative electrode active material and a binder and conductive material selectively in a solvent, onto a negative electrode current collector and then drying it, or by casting the negative electrode slurry composition onto a separate support, peeling it off the support, and then laminating the resulting film onto the negative electrode current collector.

[0163] In other embodiments, the negative electrode active material layer may be manufactured by coating a negative electrode slurry composition, prepared by dissolving or dispersing a negative electrode active material and a binder and conductive material selectively in a solvent, onto a negative electrode current collector and drying it, or by casting the negative electrode slurry composition onto a separate support, peeling it off the support, and laminating the resulting film onto the negative electrode current collector.

[0164] On the other hand, in the lithium secondary battery, the separator separates the negative electrode and the positive electrode and provides a passage for lithium ions to move. Generally, any separator commonly used in lithium secondary batteries can be used without particular limitations, and it is especially preferable that the separator has low resistance to ion movement of the electrolyte and excellent electrolyte moisture absorption capacity. Specifically, porous polymer films, such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminated structures of two or more layers thereof may be used. Alternatively, ordinary porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, may be used. Furthermore, to ensure heat resistance or mechanical strength, coated separators containing ceramic components or polymeric substances may be used, and may be selectively used as single-layer or multi-layer structures.

[0165] Furthermore, the electrolytes used in the present invention include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the manufacture of lithium secondary batteries.

[0166] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0167] The organic solvent can be any solvent that serves as a medium through which ions involved in the electrochemical reaction of the battery can move, without any particular limitations. Specifically, the organic solvent may be an ester solvent such as methyl acetate, ethyl acetate, γ-butyrolactone, or ε-caprolactone; an ether solvent such as dibutyl ether or tetrahydrofuran; a ketone solvent such as cyclohexanone; an aromatic hydrocarbon solvent such as benzene or fluorobenzene; dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), or propylene carbonate (propylene Carbonate solvents such as carbonate (PC), alcohol solvents such as ethyl alcohol and isopropyl alcohol, nitriles such as R-CN (where R is a linear, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms, and may include a double-bonded aromatic ring or ether bond), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, or sulfolanes may be used. Among these, carbonate solvents are preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and high dielectric constant that can improve the charge and discharge performance of the battery, and a low-viscosity linear carbonate compound (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) is more preferred. In this case, the electrolyte performance can be improved by mixing the cyclic carbonate and the linear carbonate in a volume ratio of about 1:1 to about 1:9.

[0168] The lithium salt may be any compound capable of providing lithium ions for use in a lithium secondary battery, without any particular limitations. Specifically, the lithium salt may be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3), LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2, etc. The concentration of the lithium salt is preferably within the range of 0.1 to 2.0 M. When the concentration of the lithium salt falls within this range, the electrolyte has appropriate conductivity and viscosity, so it can exhibit excellent electrolyte performance and lithium ions can move effectively.

[0169] In the present invention, when the electrolyte used is a solid electrolyte, a solid inorganic electrolyte such as a sulfide-based solid electrolyte, oxide-based solid electrolyte, nitride-based solid electrolyte, or halogen-based solid electrolyte may be used, and preferably a sulfide-based solid electrolyte may be used.

[0170] As the material for the sulfide-based solid electrolyte, a solid electrolyte containing Li, element X (where X is at least one selected from P, As, Sb, Si, Ge, Sn, B, Al, Ga, and In) and S may be used. Examples of the sulfide-based solid electrolyte materials include Li2S-P2S5, Li2S-P2S-LiX (where X is a halogen element such as I or Cl), Li2S-P2S5-Li2O, Li2S-P2S5-Li2O-LiI, Li2S-SiS2, Li2S-SiS2-LiI, Li2S-SiS2-LiBr, Li2S-SiS2-LiCl, Li2S-SiS2-B2S3-LiI, Li2S-SiS2-P2S5-LiI, Li2S-B2S3, Li2S-P2S5-ZmSn (where m and n are integers and Z is Ge, Zn, or Ga), Li2S-GeS2, Li2S-SiS2-Li3PO4, and Li2S-SiS2-Li p MO q(Here, p and q are integers, and M is P, Si, Ge, B, Al, Ga, or In.)

[0171] The solid electrolyte, preferably a sulfide-based solid electrolyte, may be amorphous or crystalline, or a mixture of amorphous and crystalline materials.

[0172] As a material for oxide-based solid electrolytes, Li7La3Zr2O 12 Li 7-x La3Zr 1-x Nb x O 12 Li 7-3x La3Zr2Al x O 12 Li 3x La 2 / 3-x TiO3, Li 1+x Al x Ti 2-x (PO4)3, Li 1+x Al x Ge 2-x (PO4)3, Li3PO4, Li 3+x PO 4-x N x (LiPON), Li 2+2x Zn 1-x Examples include GeO4 (LISICON).

[0173] The aforementioned solid electrolyte may be arranged as a separate layer (solid electrolyte layer) between the positive electrode and the negative electrode. Furthermore, the solid electrolyte may be partially included in the positive electrode active material layer of the positive electrode independently of the solid electrolyte layer, or partially included in the negative electrode active material layer of the negative electrode independently of the solid electrolyte layer.

[0174] In addition to the electrolyte components, the electrolyte may further contain one or more additives for the purpose of improving battery life characteristics, suppressing the decrease in battery capacity, and improving battery discharge capacity, such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphate triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride. In this case, the additive may be present in an amount of 0.1 to 5 wt% relative to the total weight of the electrolyte.

[0175] As described above, the lithium secondary battery containing the positive electrode active material according to the present invention exhibits excellent discharge capacity, output characteristics, and life characteristics in a stable manner, making it useful in portable devices such as mobile phones, laptop computers, and digital cameras, as well as in the electric vehicle field, such as hybrid electric vehicles (HEVs).

[0176] The external shape of the lithium secondary battery according to the present invention is not particularly limited, but it may be cylindrical, rectangular, pouch-shaped, or coin-shaped using a can. Furthermore, the lithium secondary battery can be used not only as a battery cell used as a power source for small devices, but may also be preferably used as a unit battery in medium-to-large battery modules containing multiple battery cells.

[0177] According to yet another aspect of the present invention, a battery module and / or a battery pack including the lithium secondary battery as a unit cell can be provided.

[0178] The battery module or battery pack can be used as a power source for one or more medium-to-large devices, including power tools, electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs), or power storage systems.

[0179] The present invention will be described in more detail below with reference to examples. However, these examples are for illustrative purposes only and should not be construed as limiting the scope of the present invention.

[0180] Manufacturing Example 1. Manufacturing of positive electrode active material Manufacturing Example 1-1. Production of Lithium Manganese Oxide No. 1 (A-1) (a) Production of precursors An aqueous solution of NiSO4·6H2O and MnSO4·H2O mixed in a molar ratio of 40:60, along with NaOH and NH4OH, was added to the reactor while stirring. The reactor temperature was maintained at 45°C, and the precursor synthesis reaction was carried out while N2 gas was introduced into the reactor. After the reaction was complete, the product was washed and dehydrated, and Ni 0.4 Mn 0.6 A hydroxide precursor with an (OH)2 composition (average particle size 3.0 μm; known methods can be used to adjust the particle size) was obtained.

[0181] (b) First heat treatment After heating an O2 atmosphere furnace at a rate of 2°C / min, the temperature was maintained at 550°C, and the hydroxide precursor obtained in step (a) was heat-treated for 5 hours, followed by furnace cooling to obtain the oxide state precursor.

[0182] (c) Second heat treatment The oxide precursor obtained in step (b) above and the lithium raw material LiOH (Li / Metal molar ratio = 1.25) were mixed to prepare a mixture. Next, the mixture was heat-treated in an O2 atmosphere furnace at a rate of 2°C / min, maintained at 900°C for 8 hours, and then furnace-cooled to obtain the first lithium manganese oxide (A-1).

[0183] Manufacturing Example 1-2. Manufacturing of Lithium Manganese Oxide No. 1 (A-2) (a) Production of precursors An aqueous solution of NiSO4·6H2O and MnSO4·H2O mixed in a molar ratio of 40:60, along with NaOH and NH4OH, was added to the reactor while stirring. The reactor temperature was maintained at 45°C, and the precursor synthesis reaction was carried out while N2 gas was introduced into the reactor. After the reaction was complete, the product was washed and dehydrated, and Ni 0.4 Mn 0.6 A hydroxide precursor with an (OH)2 composition (average particle size 3.0 μm; known methods can be used to adjust the particle size) was obtained.

[0184] (b) First heat treatment After heating an O2 atmosphere furnace at a rate of 2°C / min, the temperature was maintained at 550°C, and the hydroxide precursor obtained in step (a) was heat-treated for 5 hours, followed by furnace cooling to obtain the oxide state precursor.

[0185] (c) Second heat treatment A mixture was prepared by mixing the oxide precursor obtained in step (b) above, LiOH (Li / Metal molar ratio = 1.25), a lithium raw material, LiF weighed so that the content of fluorine (F) on a metallic basis, excluding lithium in the precursor, is 1.0 mol%, and WO3 weighed so that the content of tungsten (W) on a metallic basis, excluding lithium in the precursor, is 0.75 mol%. Next, the mixture was heat-treated in an O2 atmosphere furnace at a rate of 2°C / min, maintained at 900°C for 8 hours, and then furnace-cooled to obtain the first lithium manganese oxide (A-2).

[0186] Manufacturing Example 1-3. Production of Lithium Manganese-based Oxide (B-1) (a) Production of precursors An aqueous solution of NiSO4·6H2O and MnSO4·H2O mixed in a molar ratio of 40:60, along with NaOH and NH4OH, was added to the reactor while stirring. The reactor temperature was maintained at 45°C, and the precursor synthesis reaction was carried out while N2 gas was introduced into the reactor. After the reaction was complete, the product was washed and dehydrated, and Ni 0.4 Mn 0.6 A hydroxide precursor with an (OH)2 composition (average particle size 12.0 μm; known methods can be used to adjust the particle size) was obtained.

[0187] (b) First heat treatment After heating an O2 atmosphere furnace at a rate of 2°C / min, the temperature was maintained at 550°C, and the hydroxide precursor obtained in step (a) was heat-treated for 5 hours, followed by furnace cooling to obtain the oxide state precursor.

[0188] (c) Second heat treatment The oxide precursor obtained in step (b) above and the lithium raw material LiOH (Li / Metal molar ratio = 1.25) were mixed to prepare a mixture. Next, the mixture was heat-treated in an O2 atmosphere furnace at a rate of 2°C / min, maintained at 900°C for 8 hours, and then furnace-cooled to obtain a second lithium manganese oxide (B-1).

[0189] Manufacturing Example 1-4. Production of Lithium Manganese Oxide II (B-2) (a) Production of precursors An aqueous solution of NiSO4·6H2O and MnSO4·H2O mixed in a molar ratio of 40:60, along with NaOH and NH4OH, was added to the reactor while stirring. The reactor temperature was maintained at 45°C, and the precursor synthesis reaction was carried out while N2 gas was introduced into the reactor. After the reaction was complete, the product was washed and dehydrated, and Ni 0.4 Mn 0.6 A hydroxide precursor with an (OH)2 composition (average particle size 12.0 μm; known methods can be used to adjust the particle size) was obtained.

[0190] (b) First heat treatment After heating an O2 atmosphere furnace at a rate of 2°C / min, the temperature was maintained at 550°C, and the hydroxide precursor obtained in step (a) was heat-treated for 5 hours, followed by furnace cooling to obtain the oxide state precursor.

[0191] (c) Second heat treatment A mixture was prepared by mixing the oxide precursor obtained in step (b) above, LiOH (Li / Metal molar ratio = 1.25), a lithium raw material, LiF weighed so that the content of fluorine (F) on a metallic basis, excluding lithium in the precursor, is 1.0 mol%, and WO3 weighed so that the content of tungsten (W) on a metallic basis, excluding lithium in the precursor, is 0.75 mol%. Next, the mixture was heat-treated in an O2 atmosphere furnace at a rate of 2°C / min, maintained at 900°C for 8 hours, and then furnace-cooled to obtain a second lithium manganese oxide (B-2).

[0192] Manufacturing Example 2. Manufacturing of Cathode Active Material The first lithium manganese oxide and the second lithium manganese oxide produced by Production Example 1 were mixed in the weight ratios shown in Table 1 below to produce the positive electrode active material.

[0193] [Table 1]

[0194] Manufacturing Example 3: Manufacturing of Lithium-ion Rechargeable Batteries A cathode slurry was prepared by dispersing 90 wt% of the cathode active material, 4.5 wt% of carbon black, and 5.5 wt% of the PVDF binder produced by manufacturing example 2 in N-methyl-2-pyrrolidone (NMP). The cathode slurry was uniformly coated onto a 15 μm thick aluminum thin film and vacuum-dried at 135°C to produce a cathode for a lithium secondary battery.

[0195] A half-cell was manufactured using a lithium foil as the counter electrode for the positive electrode, a porous polyethylene membrane (Celgard 2300, thickness: 25 μm) as the separator, and an electrolyte containing ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate mixed in a volume ratio of 2:4:4, with LiPF6 present at a concentration of 1.15 M.

[0196] Experimental Example 1. Image analysis of lithium manganese oxides (1) After selecting secondary particles from each lithium manganese oxide produced by Production Examples 1-1 to 1-4, scanning electron microscope (SEM) images were obtained.

[0197] Next, using the Image analyzer program, one secondary particle was selected from the SEM image, and 20 primary particles were selected from the surface of that secondary particle in descending order of their short axis length, and the long axis length and short axis length of each were measured. From the measurement results, the average value A of the short axis length of the primary particle, the maximum value B of the short axis length of the primary particle, the minimum value C of the short axis length of the primary particle, the average value D of the particle size of the primary particle, the maximum value E of the particle size of the primary particle, and the minimum value F of the particle size of the primary particle were calculated, respectively. The particle size of the primary particle was calculated as the average of the long axis length and short axis length of the primary particle ([long axis length + short axis length] / 2).

[0198] (2) Each of the selected secondary particles was then cross-section polished using a cross-section polisher (acceleration voltage 5.0kV, 4-hour milling), and then scanned with a scanning electron microscope to obtain cross-sectional SEM images.

[0199] Next, using an Image analyzer program, the number of pores G was counted from the cross-sectional SEM image of the secondary particle, and the cross-sectional porosity H was calculated.

[0200] The cross-sectional porosity H was calculated by binarizing the cross-sectional SEM image and then determining the ratio of the total area of ​​all voids within the secondary particle to the total area of ​​the cross-section of the secondary particle ([total area of ​​all voids within the secondary particle / total area of ​​the cross-section of the secondary particle] / 100).

[0201] Furthermore, the center of the secondary particle was determined from the bimodal cross-sectional SEM image, and the distance from the center of the secondary particle to the surface of the secondary particle was denoted as r. The region from the center of the secondary particle at a distance of 0.5r to 1.0r was then defined as the outer bulk region.

[0202] The external bulk porosity (I) ([total area of ​​all voids in the external bulk region / total area of ​​the external bulk region] × 100) was calculated by dividing the total cross-sectional area of ​​the external bulk region by the total area of ​​all voids present within the external bulk region.

[0203] The measurement results are shown in Table 2 below.

[0204] [Table 2]

[0205] Experimental Example 2. Measurement of Compression Density of Cathode Active Material Three g of each positive electrode active material produced according to Production Example 2 was pressurized for 5 seconds using a pelletizer under the pressure conditions listed in Table 3 below, and then its compressed density was measured.

[0206] The measurement results are shown in Table 3 below.

[0207] [Table 3]

[0208] Referring to the results of Comparative Examples 1 to 3, it can be confirmed that the 2.5-ton compressed density of Comparative Example 3 (bimodal type cathode active material including A-1 and B-1) is slightly higher than that of Comparative Example 1 (unimodal type cathode active material including A-2) and Comparative Example 2 (unimodal type cathode active material including B-2), but it can be confirmed that there is no significant difference in the 4.5-ton compressed density between Comparative Example 2 and Comparative Example 3.

[0209] On the other hand, referring to the results of Examples 1 to 6, in which at least one selected from the first lithium manganese oxide and the second lithium manganese oxide contains secondary particles in the form of aggregated large-particle primary particles, it can be confirmed that there is a significant improvement in both the 2.5-ton and 4.5-ton compressed density compared to Comparative Examples 1 to 3.

[0210] Experimental Example 3. Evaluation of the electrochemical properties of lithium secondary batteries For lithium secondary batteries (half-cells) manufactured in Manufacturing Example 3, the initial charge capacity, initial discharge capacity, capacity per unit volume, initial reversibility efficiency, and rate characteristics (rate capability (C-rate)) were measured through charge-discharge experiments using an electrochemical analyzer (Toyo, Toscat-3100) at 25°C, with a voltage range of 2.0V to 4.6V and a discharge rate of 0.1C to 5.0C. The capacity per unit volume was calculated by multiplying the initial discharge capacity by the compressed density (compressed density of 4.5 tonns in Table 3).

[0211] Furthermore, the same lithium secondary battery underwent 50 charge-discharge cycles at 25°C and within a driving voltage range of 2.0V to 4.6V under 1C / 1C conditions. The ratio of the discharge capacity at the 50th cycle to the initial discharge capacity (cycle capacity retention) was then measured.

[0212] The measurement results are shown in Table 4 below.

[0213] [Table 4]

[0214] Referring to the results in Table 4, and the results in Examples 1 to 6, which include secondary particles in the form of aggregated large-particle primary particles, it can be confirmed that the volume per unit volume is improved compared to Comparative Examples 1 to 3.

[0215] Although embodiments of the present invention have been described above, a person with ordinary skill in the art can modify and change the present invention in various ways, such as by adding, changing, deleting, or adding components, without departing from the spirit of the invention as described in the claims, and this can also be said to be within the scope of the rights of the present invention.

Claims

1. A bimodal type positive electrode active material comprising a first lithium manganese oxide as small particles and a second lithium manganese oxide as large particles, The first lithium manganese oxide and the second lithium manganese oxide are oxides in which a phase belonging to the C2 / m space group and a phase belonging to the R3-m space group are in solid solution or composite form. The first lithium manganese oxide and the second lithium manganese oxide each independently contain at least one form of secondary particles selected from a form in which multiple large-sized primary particles are aggregated and a form in which multiple small-sized primary particles are aggregated, At least one selected from the first lithium manganese oxide and the second lithium manganese oxide includes secondary particles in the form of aggregated large primary particles and secondary particles formed by the aggregation of multiple small primary particles. The large-sized primary particles and the small-sized primary particles each have a long axis and a short axis, The average value of the short axis length of the aforementioned large-sized primary particles is between 100 nm and 500 nm. The average value of the short axis length of the aforementioned small-sized primary particles is 50 nm or more and 300 nm or less, A positive electrode active material wherein the average value of the short axis length of the small-sized primary particles is smaller than the average value of the short axis length of the large-sized primary particles.

2. The positive electrode active material according to claim 1, wherein the first lithium manganese-based oxide contains secondary particles in the form of aggregated large primary particles and secondary particles in the form of aggregated small primary particles in a weight ratio of 10:90 to 100:

0.

3. The first lithium manganese oxide includes secondary particles in which the plurality of large-sized primary particles are aggregated and secondary particles in which the plurality of small-sized primary particles are aggregated. The positive electrode active material according to claim 2, wherein the interparticle porosity between primary particles, measured from a cross-sectional SEM image of secondary particles in which the large-particle-sized primary particles are aggregated, is smaller than the interparticle porosity between primary particles, measured from a cross-sectional SEM image of secondary particles in which the small-particle-sized primary particles are aggregated.

4. The positive electrode active material according to claim 3, wherein the interparticle porosity between the primary particles, measured from a cross-sectional SEM image of secondary particles in which the large-particle-sized primary particles are aggregated, is 10% or less.

5. The first lithium manganese oxide includes secondary particles in which the plurality of large-sized primary particles are aggregated and secondary particles in which the plurality of small-sized primary particles are aggregated. When the distance from the center of the secondary particle to the surface of the secondary particle, as determined from the cross-sectional SEM image of the secondary particle, is called r, and the region between 0.5r and 1.0r from the center of the secondary particle is called the external bulk region, The positive electrode active material according to claim 2, wherein the porosity in the outer bulk region measured from a cross-sectional SEM image of secondary particles in a form in which the large-particle-sized primary particles are aggregated is smaller than the porosity in the outer bulk region measured from a cross-sectional SEM image of secondary particles in a form in which small-particle-sized primary particles are aggregated.

6. The positive electrode active material according to claim 5, wherein the porosity in the outer bulk region, as measured from a cross-sectional SEM image of secondary particles in a form in which the large-particle-sized primary particles are aggregated, is 1% or less.

7. The positive electrode active material according to claim 1, wherein the average particle size of the first lithium manganese-based oxide is 2 μm to 5 μm.

8. The positive electrode active material according to claim 1, wherein the second lithium manganese-based oxide contains secondary particles in the form of aggregated large primary particles and secondary particles in the form of aggregated small primary particles in a weight ratio of 10:90 to 100:

0.

9. The second lithium manganese oxide includes secondary particles in which the plurality of large-sized primary particles are aggregated and secondary particles in which the plurality of small-sized primary particles are aggregated. The positive electrode active material according to claim 7, wherein the interparticle porosity between primary particles, measured from a cross-sectional SEM image of secondary particles in which the large-particle-sized primary particles are aggregated, is smaller than the interparticle porosity between primary particles, measured from a cross-sectional SEM image of secondary particles in which the small-particle-sized primary particles are aggregated.

10. The positive electrode active material according to claim 9, wherein the interparticle porosity between the primary particles, as measured from a cross-sectional SEM image of secondary particles in which the large-particle-sized primary particles are aggregated, is 15% or less.

11. The second lithium manganese oxide includes secondary particles in which the plurality of large-sized primary particles are aggregated and secondary particles in which the plurality of small-sized primary particles are aggregated. When the distance from the center of the secondary particle to the surface of the secondary particle, as determined from the cross-sectional SEM image of the secondary particle, is called r, and the region between 0.5r and 1.0r from the center of the secondary particle is called the external bulk region, The positive electrode active material according to claim 7, wherein the porosity in the outer bulk region measured from a cross-sectional SEM image of secondary particles in a form in which the large-particle-sized primary particles are aggregated is smaller than the porosity in the outer bulk region measured from a cross-sectional SEM image of secondary particles in a form in which small-particle-sized primary particles are aggregated.

12. The positive electrode active material according to claim 11, wherein the porosity in the outer bulk region, measured from a cross-sectional SEM image of secondary particles in a form in which the large-particle-sized primary particles are aggregated, is 6% or less.

13. The positive electrode active material according to claim 1, wherein the average particle size of the second lithium manganese oxide is 6 μm to 14 μm.

14. The positive electrode active material according to claim 1, wherein the first lithium manganese oxide and the second lithium manganese oxide are contained in the positive electrode active material in a weight ratio of 10:90 to 80:

20.

15. The positive electrode active material according to claim 1, wherein the first lithium manganese oxide and the second lithium manganese oxide each independently comprise at least one selected from nickel, cobalt, and manganese.

16. The positive electrode active material according to claim 1, wherein at least one selected from the first lithium manganese oxide and the second lithium manganese oxide comprises secondary particles doped with at least one dopant selected from metal cation dopants and halogen anion dopants.

17. The positive electrode active material according to claim 16, wherein the metal cation dopant is at least one selected from W, Mo, and Nb.

18. The positive electrode active material according to claim 16, wherein the halogen anion dopant is fluorine.

19. The positive electrode active material according to claim 16, wherein the secondary particles in the form of aggregated large-particle primary particles are doped with at least one dopant selected from metal cation dopants and halogen anion dopants.

20. The positive electrode active material according to claim 1, wherein the first lithium manganese oxide and the second lithium manganese oxide are each independently represented by the following chemical formula 1. [Chemical formula 1] rLi ２ MnO ３－ｂ′ X ｂ′ ・(1-r)Li ａ M1 ｘ M2 ｙ M3 ｚ O ２－ｂ X ｂ (Here, M1 is at least one selected from Ni and Mn. M2 is at least one selected from Ni, Mn, Co, Al, P, Nb, B, Si, Ti, Zr, Ba, K, Mo, Fe, Cu, Cr, Zn, Na, Ca, Mg, Pt, Au, Eu, Sm, W, Ce, V, Ta, Sn, Hf, Gd, and Nd. M3 is at least one selected from W, Mo, and Nb. M1 to M3 do not overlap with each other. X and X' are halogens capable of substituting at least a portion of the oxygen present in the first lithium manganese oxide and the second lithium manganese oxide, (0 < r ≤ 0.7, 0 < a ≤ 1, 0 ≤ b ≤ 0.1, 0 ≤ b' ≤ 0.1, 0 < x ≤ 1, 0 ≤ y < 1, 0 ≤ z ≤ 0.1, and 0 < x + y + z ≤ 1)

21. A positive electrode comprising the positive electrode active material according to any one of claims 1 to 20.

22. A lithium secondary battery using the positive electrode described in claim 21.

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